1. Field of the Invention
This invention relates to an evaporator into which a reactive gas is introduced to perform a vacuum deposition, and particularly to a reaction deposition evaporator.
2. State of the Art
This type of evaporator can be used to, for example, prepare a transparent conductive film consisting of indium tin oxide (i.e., ITO), where at least one of an In--Sn alloy and the oxides thereof is evaporated and deposited on a substrate in the presence of an ionized or activated oxygen gas.
Examples of this type of evaporators are a D.C. ion-plating apparatus and an RF ion-plating apparatus. Examples of the RF ion-plating utilizing high-frequency discharge have been described in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 84474/1975, 113733/1974, and 120877/1974.
As shown in FIG. 1, for example, substrate 2 is heated in a vacuum chamber (bell-jar) 30 by heater 34 and a prescribed voltage 35 from -10 V to -10 kV is applied, if necessary, to an electrode behind substrate 2. A.C. voltage 71 is applied to coil electrode 70 which is a high frequency discharge electrode arranged over a vapor source 25. The A.C. frequency may be freely selected, however, a discharge is stabilised when using a high frequency such as 13.56 MHz. A glow discharge generated thereby between substrate 2 and electrode 70 activates or ionizes vapor of an evaporated material and each atom of unactivated reactive gas introduced from gas introduction tube 36 so that an ITO layer may be accumulated on substrate 2.
It was, however, found that the described well-known apparatuses have unacceptable disadvantages as mentioned in the following items (1) and (2):
(1) A high frequency discharge is stably performed in such a low vacuum state as the order of 10.sup.-3 -10.sup.-4 Torr. However, in a high vacuum state such as 10.sup.-5 -10.sup.-4 Torr, the discharge will be unstable, so that it will become difficult to perform a reaction deposition.
Accordingly, to increase the adhesion of a deposited layer to a substrate, and to form the deposited layer with a close-packed structure, it is necessary to apply an electric field for accelerating evaporated material by applying a high voltage to the substrate. It is, however, difficult to provide a voltage application means when a deposition layer is formed on an insulating substrate such as a glass or plastic substitute.
(2) Because of a large amount of evaporated materials become deposited on a high frequency discharge electrode, the deposited evaporated materials are peeled off after being bombarded by discharge during a long period of operation. Thereby, the discharge becomes unstable or the evaporated materials get mixed in a vapor source so that the deposited layer becomes uneven. Further, the evaporated materials deposit on a high frequency lead-in terminal to cause a leak.
A conventional discharger 37 shown in FIG. 6 comprises an electrode 62', which is one end of a cylinder having gas inlet 61', and a discharge space 63' having a member 64', comprising an annular glass structure for example, joined at one end thereof with electrode 62' so as to enclose discharge space 63'. Another ring-shaped electrode 66' has an outlet 65' at the other end of the above-mentioned charge space enclosing member 64'. A D.C. or A.C. voltage is applied between electrode 62' and electrode 66', and a glow discharge thereby occurs in discharge space 63' with hydrogen gas, for example, supplied through gas inlet 61'. Active hydrogen comprising hydrogen atoms or molecules electronically energized by the glow-discharge and hydrogen ions ionized thereby are exhausted from outlet 65'. The exemplified discharge space enclosing member 64' is of a double tube, i.e., annular, structure that permits cooling water to flow therethrough. The numerals 67', 68', respectively, designate the cooling water inlet and outlet. The numeral 69' designates the cooling fin for electrode 62'.
In the hydrogen gas discharge tube of FIG. 6, the distance between the electrodes is 10 cm to 15 cm, and a voltage of 600 V is applied. The pressure in discharge space 63' is on the order of 10.sup.-2 Torr. In this discharger, however, electrodes 62', 66' and the sealing member (not shown) provided in the electrode fitting position are connected to discharge region 63'. Therefore, they are bombarded by gas ions produced when discharging, and contaminations, sealing deterioration and the like are produced, so that the discharger is impractical to use.